Nuclear factor kappa B (NF-?B) has been illustrated as an important transcription factor in a variety of cellular processes and the NF-?B signaling pathway has provided a focus for pharmacological intervention. NF-?B is responsive to genotoxic threats such as DNA damaging chemicals and ionizing irradiation. Moreover, the NF-?B signaling pathway has emerged as one of the most important mediators of the cellular responses to genotoxic stress, and considerable progress has been made during the past decade to understand DNA damage responses; that said, the ''nuclear-to-cytoplasmic'' NF-?B signaling pathways following various genotoxic stresses remain less defined, in particular the upstream signaling cascade that networks nuclear DNA damage and cytoplasmic NF-?B activation. Our recent study revealed that Sam68 (Src-associated substrate during mitosis of 68 kDa) is critical for the DNA damage-triggered NF-?B activation and regulates genotoxic stress-induced poly(ADP-ribosyl)ation (PARylation). This project aims to elucidate the mechanisms how Sam68 functions in the DNA damage-initiated signaling pathway, and to assess the pathophysiological significance of Sam68-dependent NF-?B signaling in primary cell cultures, whole animals, and human cancer cells. We will determine the role of Sam68 in genotoxic stress-initiated NF-?B signaling pathway in Aim 1 and elucidate the mechanism(s) how Sam68 regulates PARylation following DNA damage in Aim 2. Furthermore, we will assess the role of Sam68 in genotoxic stress-induced NF-?B signaling and activation in mouse primary cells, whole animals, and human cancer cells in Aim 3. At the conclusion of these studies, we will elucidate novel molecular mechanisms on DNA damage-initiated upstream signaling cascade that leads to NF-?B activation, thus advancing our fundamental understanding of this important nuclear-to-cytoplasmic NF-?B signaling pathway. It may also lead to novel targets that may aid rational drug development for the genotoxic stress- and NF-?B-associated diseases.